The present invention relates to the field of the production of multilayer semiconductor wafers or substrates produced by transfer of at least one layer formed from an initial substrate onto a final substrate, the layer transferred corresponding to a portion of the initial substrate. The layer transferred may further comprise all or part of a component or of a plurality of microcomponents.
The present invention concerns the problem of the heterogeneous deformations that occur in a layer bonded by molecular adhesion to a substrate, and to be more precise on the transfer of such a layer from an initial substrate called the “donor substrate” to a final substrate called the “receiver substrate”. Such deformations have notably been observed in the case of the technology of three-dimensional integration of components (3D-integration) which necessitates the transfer of one or more layers of microcomponents onto a final support substrate but also in the case of transfer of circuits or in the production of back-lit imaging devices. Because in particular of the very small size and the large number of microcomponents generally present on the layers transferred, each of them must be positioned on the final substrate with great accuracy in order to comply with a very strict alignment with the underlying layer. Moreover, it may be necessary to carry out treatments on the layer after its transfer, for example to form other microcomponents, to uncover microcomponents on the surface, to make interconnections, etc.
However, the Applicant has noticed that after such a transfer there exist situations in which it is very difficult or even impossible to form additional microcomponents in alignment with the microcomponents formed before the transfer.
This phenomenon of misalignment is described with reference to FIGS. 1A to 1E which show one embodiment of a three-dimensional structure comprising the transfer onto a final substrate of a layer of microcomponents formed on an initial substrate and the formation of an additional layer of microcomponents on the exposed face of the initial substrate after bonding.
FIGS. 1A and 1B show an initial substrate 10 on which is formed a first series of microcomponents 11. The microcomponents 11 are formed by photolithography by means of a mask enabling definition of the areas of formation of patterns corresponding to the microcomponents 11 to be produced.
As shown in FIG. 1C, the face of the initial substrate 10 comprising the microcomponents 11 is then brought into intimate contact with a face of a final substrate 20, thus forming the composite structure 25. The bonding between the initial substrate 10 and the final substrate 20 is effected by molecular adhesion. There is thus obtained a buried layer of microcomponents 11 at the bonding interface between the substrates 10 and 20. After bonding, and as shown in FIG. 1D, the initial substrate 10 is thinned in order to remove a portion of material present over the layer of microcomponents 11. A thinned composite structure 30 is then obtained formed of the final substrate 20 and a layer 10a corresponding to the remaining portion of the initial substrate 10.
As shown in FIG. 1E, the next step in the production of the three-dimensional structure consists in forming a second layer of microcomponents 12 at the level of the exposed surface of the thinned initial substrate 10, or in carrying out complementary technological steps on this exposed surface, in alignment with the components included in the layer 10a (contacts, interconnections, etc.). For simplicity, in the remainder of this text the term “microcomponents” refers to devices or any other patterns resulting from technology steps effected on or in the layers and the positioning of which must be controlled accurately. It may thus be a question of active or passive components, contacts or interconnections.
Thus, in order to form the microcomponents 12 in alignment with the buried microcomponents 11, a photolithographic mask is used similar to that used to form the microcomponents 11. Here similar masks means masks that were designed to be used in association during a fabrication process.
The layers transferred, such as the layer 10a, typically comprise marks (or markers) both at the level of the microcomponents and at the level of the slice forming the layer that are notably used by positioning and alignment tools during technological treatment steps such as those executed for the purposes of photolithography.
However, even if positioning tools are used, offsets arise between some of the microcomponents 11 and 12, such as the offsets Δ11, Δ22, Δ33, Δ44 indicated in FIG. 1E (respectively corresponding to the offsets observed between the pairs of microcomponents 111/121, 112/122, 113/123 and 114/124).
These offsets are not the result of basic transformations (translation, rotation or combinations thereof) that could originate in inaccurate assembly of the substrates. These offsets result from heterogeneous deformations that occur in the layer coming from the initial substrate when it is assembled with the final substrate. These deformations lead to local and non-uniform movements at the level of some microcomponents 11. Also, some of the microcomponents 12 formed on the exposed surface 14b of the substrate after transfer feature variations of position with these microcomponents 11 that may be of the order of several hundred nanometres or even one micron.
The phenomenon of so-called “overlay” or misalignment between the two layers of microcomponents 11 and 12 may be the source of short circuits, distortions in the stack or connection faults between the microcomponents of the two layers. Thus if the microcomponents transferred are imagers formed of pixels and the post-transfer processing steps are aimed at forming colour filters on each of these pixels, there has been observed a loss of the colorization function for some of these pixels.
This misalignment phenomenon thus leads to a reduction in the quality and the value of the multilayer semiconductor wafers produced. The impact of this phenomenon becomes more and more critical because of the ever increasing requirements in respect of miniaturization of microcomponents and their integration density in each layer.
The method routinely used nowadays to determine if significant heterogeneous deformations are present in a multilayer wafer consists in determining the positioning of a number of microcomponents by carrying out optically measurements of position at the level of markers formed on or in the vicinity of those microcomponents (verniers, etc.).
However, it is possible to proceed to these positioning tests only after thinning the initial substrate and carrying out complementary technological steps on the exposed surface 14b of the initial substrate 10.
Moreover, if alignment defects are detected in the initial substrate after it is thinned, they cannot be corrected. In this case, the thinned initial substrate cannot be recycled. In the final analysis, if positioning tests reveal in a thinned composite structure misalignments that are unacceptable in terms of reliability and/or performance, the final substrate is lost, which significantly increases the cost of production of multilayer wafers.
A technique for determination of misalignments in a semiconductor wafer is moreover described in the patent document WO 2007/103566 A2. To be more precise, this technique aims to evaluate misalignments liable to occur in a wafer during a photolithography step, these misalignments resulting from mechanical stresses generated in the wafer.
In practise, this technique consists in carrying out curvature measurements on one face of a layer produced by deposition on a substrate. From curvature data obtained at different points of the layer, the internal mechanical stresses of this layer relative to the substrate are determined. Knowing these stresses, it is possible to evaluate the “movements” of this layer relative to the substrate. The evaluation of these movements before or during a photolithography step notably makes it possible to determine how to compensate or correct the photolithography parameters in such a manner as to minimize the misalignments.
However, this technique concerns only the evaluation of deformations generated over the whole of a layer produced by deposition on a substrate (or possibly by ion implantation, annealing or etching). These so-called homogeneous deformations are in fact the result of a mechanical equilibrium obtained over the whole of the layer deposited on the substrate. This type of deformation exhibits a behaviour that is now relatively predictable thanks to the use of models taking into account in particular the laws of mechanics and the thicknesses in play (cf. equation 5 on page 5 of the document WO 2007/103566 A2).
The technique described in the document WO 2007/103566 A2 is, not designed to evaluate heterogeneous deformation resulting from bonding of two wafers, however, and in particular bonding of molecular adhesion type the mechanisms whereof are still very badly understood at present.
The Applicant has noticed that the behaviour of the heterogeneous deformations resulting from bonding by molecular adhesion is random and in any event very different from the classic homogeneous deformations. At this writing, no model enables reliable evaluation of the level of heterogeneous deformations generated in a layer bonded by molecular adhesion to a substrate. There therefore exists a requirement to evaluate in a simple and effective manner the level of heterogeneous deformations in multilayer structures produced by bonding by molecular adhesion, at an earlier stage of their fabrication.